In a conventional package-on-package (POP) process, a top package, in which a first device die is bonded, is bonded to a bottom package. The bottom package may also have a device die packaged therein. By adopting the PoP process, the integration level of the packages may be increased.
Since each of the top package and the bottom package includes different materials that have different coefficients of thermal expansion (CTEs), after the formation of the top package and the bottom package, warpage may occur in either or both of the top and bottom packages. A warpage can be a positive warpage, wherein the central portion of the package is higher than the edge portions. Conversely, a warpage can be a negative warpage, wherein the central portion of the warpage is lower than the edge portions. After the bonding of the top and the bottom packages, cold joints may occur as a result of the warpage, and some of the joints may fail. The situation is worsened when the warpage of the top package does not match the warpage of the bottom package. For example, when the top package has a positive warpage and the bottom package has a negative warpage, or the top package has a negative warpage and the bottom package has a positive warpage, the chance of the occurrence of the cold joints increases.